The present invention relates generally to a method and apparatus for detecting a planarized outer layer of a semiconductor wafer, and more particularly to a method and apparatus for detecting a planarized outer layer of a semiconductor wafer by monitoring movement of an objective lens associated with a confocal optical system during polishing of the semiconductor wafer.
Semiconductor integrated circuits are typically fabricated by a layering process in which several layers of material are fabricated on or in a surface of a wafer, or alternatively, on a surface of a previous layer. This fabrication process typically requires subsequent layers to be fabricated upon a smooth, planar surface of a previous layer. However, the surface topography of layers may be uneven due to an uneven topography associated with an underlying layer. As a result, a layer may need to be polished in order to present a smooth, planar surface for a subsequent processing step. For example, a layer may need to be polished prior to formation of a conductor layer or pattern on an outer surface of the layer.
In general, a semiconductor wafer may be polished to remove high topography and surface defects such as scratches, roughness, or embedded particles of dirt or dust. The polishing process typically is accomplished with a polishing system that includes top and bottom platens (e.g. a polishing table and a wafer carrier or holder), between which the semiconductor wafer is positioned. The platens are moved relative to each other thereby causing material to be removed from the surface of the wafer. This polishing process is often referred to as mechanical planarization (MP) and is utilized to improve the quality and reliability of semiconductor devices. The polishing process may also involve the introduction of a chemical slurry to facilitate higher removal rates, along with the selective removal of materials fabricated on the semiconductor wafer. This polishing process is often referred to as chemical mechanical planarization or chemical mechanical polishing (CMP).
In these polishing processes, it is often important to determine when an outer layer or film has been polished to a desired planarity level. In particular, it is desirable to know when the outer layer of the semiconductor wafer has been polished to a planarity level which is acceptable for presentation of the wafer to a subsequent fabrication process.
In order to determine when a wafer has been polished to a desired planarity level, systems and techniques have heretofore been utilized which polish the wafer down to a predetermined thickness. For example, a typical method employed for determining when the wafer has been polished down to a predetermined thickness is to measure the amount of time needed to planarize a first wafer to the desired thickness, and thereafter polishing the remaining wafers for a similar amount of time. In practice this method is extremely time consuming since machine operators must inspect each wafer (e.g. measure the thickness thereof) after polishing. In particular, it is extremely difficult to precisely control the removal rate of material since the removal rate may vary during the polishing of an individual wafer. Moreover, the removal rate may be diminished in the process of polishing a number of wafers in sequence. Yet further, such methods do not actually measure the planarity of the outer layer, but rather simply make an assumption that the outer layer has been polished to an acceptable planarity level when the wafer is polished to the desired thickness.
Another method employed for determining if the wafer has reached the desired thickness is to impinge a light beam, such as a laser light beam, onto the semiconductor wafer in order to determine the thickness of the wafer. Various techniques have been used to detect when an outer film associated with the semiconductor wafer reaches the desired thickness. For example, the apparatus disclosed in U.S. Pat. No. 5,151,584 issued to Ebbing et al directs an incident laser beam onto the surface of a semi-transparent thin film (e.g. silicon dioxide) of a semiconductor wafer during etching thereof. A first portion of the incident beam is reflected from the top surface of the film, and a second portion of the incident beam is reflected from the bottom surface of the film. Since the film has a finite thickness, the two reflections will either constructively or destructively interfere with one another. As the layer is etched, its thickness is changed thereby cycling intensity of the reflected beam through constructive and destructive interference patterns which may be utilized to determine when the wafer has been etched to the desired thickness. Such a technique has a number of drawbacks associated therewith. For example, such a technique may only be utilized after certain steps in the fabrication process. For example, such a technique may be useful for measuring thickness of a blank wafer, but has been found to perform unsatisfactorily when utilized to measure thickness of a patterned wafer. Moreover, similarly to the manual inspection method discussed above, such a technique does not actually measure the planarity of the outer layer, but rather simply makes an assumption that the outer layer has been polished to an acceptable planarity level when the wafer is etched down to the desired thickness.
In order to overcome the above-mentioned drawbacks associated with wafer thickness-based polishing endpoint techniques, a number of techniques have heretofore been utilized in an attempt to measure the actual planarity of the outer layer of the wafer. For example, a method which has heretofore been employed for determining when the wafer has been polished to a desired planarity level is to periodically remove the wafer from the polishing system, and thereafter measure the planarity of the wafer with an instrument such as an atomic force microscope or a profilometer. If the wafer has been polished to the desired planarity level, the wafer is released to a subsequent fabrication step. However, if the wafer has not been polished to the desired planarity level, the wafer must be placed back into the polishing system for further polishing thereof. It should be appreciated that numerous measurements may be required to reach the desired planarity level. Hence, in practice this method is extremely time consuming since machine operators must measure each wafer (i.e. measure the planarity thereof) a number of times during the polishing process.
Thus, a continuing need exists for a method and an apparatus for in situ measurement of the planarity of the outer layer of a semiconductor wafer during polishing thereof.
In accordance with a first embodiment of the present invention, there is provided a method of planarizing a first side of a semiconductor wafer with a polishing system. The method includes the step of polishing the first side of the wafer in order to remove material from the wafer. The method also includes the step of moving a lens of a confocal optical system between a number of lens positions so as to maintain focus on the first side of the wafer during the polishing step. The method further includes the step of determining a rate-of-movement value based on movement of the lens during the moving step. Moreover, the method includes the step of stopping the polishing step if the rate-of-movement value has a predetermined relationship with a movement threshold value.
Pursuant to a second embodiment of the present invention, there is provided a method of planarizing a first side of a semiconductor wafer. The method includes the step of polishing the first side of the wafer in order to remove material from the wafer. The method also includes the step of transmitting a first incident light beam from a confocal optical system during a first time period. The first incident light beam impinges on the first side of the wafer during the polishing step so as to form a first reflected light beam which is reflected from the first side of the wafer. The method further includes the step of analyzing the first reflected light beam so as to determine if the confocal optical system is focused on the first side of the wafer during the first time period. The method yet further includes the step of transmitting a second incident light beam from the confocal optical system during a second time period. The second incident light beam impinges on the first side of the wafer during the polishing step so as to form a second reflected light beam which is reflected from the first side of the wafer. The method moreover includes the step of analyzing the second reflected light beam so as to determine if the confocal optical system is focused on the first side of the wafer during the second time period. Finally, the method includes the step of stopping the polishing step if the confocal optical system is focused on the first side of the wafer during both the first time period and the second time period.
Pursuant to a third embodiment of the present invention, there is provided an apparatus for polishing a first side of a semiconductor wafer. The apparatus includes a polishing system which operates to polish the wafer. The polishing system has a polishing platen which includes a polishing surface, and a wafer carrier which is configured to engage the wafer by a second side of the wafer, and apply pressure to the wafer in order to press the wafer against the polishing surface of the polishing platen. The apparatus also includes a confocal optical system having a movable objective lens. The confocal optical system is configured to move the objective lens between a number of lens positions so as to maintain focus on the first side of the wafer during polishing of the wafer. The apparatus further includes a controller electrically coupled to the confocal optical system. The controller is configured to determine a rate-of-movement value based on movement of the objective lens during polishing of the wafer, and terminate operation of the polishing system so as to cease polishing of the wafer in response to determination that the rate-of-movement value has a predetermined relationship with a movement threshold value.
Pursuant to a fourth embodiment of the present invention, there is provided an apparatus for polishing a first side of a semiconductor wafer. The apparatus includes a polishing system which operates to polish the wafer. The polishing system has a polishing platen which includes a polishing surface. The polishing system also includes a wafer carrier which is configured to engage the wafer by a second side of the wafer and apply pressure to the wafer in order to press the wafer against the polishing surface of the polishing platen. The apparatus also includes a confocal optical system positioned such that a first incident light beam transmitted by the confocal optical system is impinged upon the first side of the wafer during a first period of time so as to form a first reflected light beam which is reflected from the first side of the wafer. The confocal optical system is also positioned such that a second incident light beam transmitted by the confocal optical system is impinged upon the first side of the wafer during a second period of time so as to form a second reflected light beam which is reflected from the first side of the wafer. Yet further, the confocal optical system is positioned such that the first and second reflected light beams are received with the confocal optical system. The apparatus also includes a controller electrically coupled to the confocal optical system. The controller is configured to analyze the first reflected light beam so as to determine if the confocal optical system is focused on the first side of the wafer during the first time period, analyze the second reflected light beam so as to determine if the confocal optical system is focused on the first side of the wafer during the second time period, and terminate operation of the polishing system so as to cease polishing of the wafer in response to determination that the confocal optical system is focused on the first side of the wafer during both the first time period and the second time period.
It is an object of the present invention to provide a new and useful method and apparatus for determining when a semiconductor wafer has been polished to a desired planarity level.
It is also an object of the present invention to provide an improved method and apparatus for determining when a semiconductor wafer has been polished to a desired planarity level.
It is yet further an object of the present invention to provide a method and apparatus for determining when a semiconductor wafer has been polished to a desired planarity level that is less mechanically complex relative to polishing systems which have heretofore been designed.
It is moreover an object of the present invention to provide a method and apparatus for determining when a semiconductor wafer has been polished to a desired planarity level that is less mechanically complex relative to polishing systems which have heretofore been designed, yet detects the planarity level of the semiconductor wafer during polishing thereof.
It is also an object of the present invention to provide a method and apparatus for determining when a semiconductor wafer has been polished to a desired planarity level which does not require chemical analysis of the slurry associated with the polishing system.
The above and other objects, features, and advantages of the present invention will become apparent from the following description and the attached drawings.